JP5927368B1 - Endoscope objective optical system - Google Patents

Abstract

It is easy to adjust the focus while making meridian, and obtains bright, wide-angle and high-definition image quality. In order from the object side, it consists of a front group (G1) with negative refractive power, an aperture stop (S), and a positive rear group (G2). The front group is a single lens with negative refractive power in order from the object side. A first lens (L1) and a second lens (L2) that is a single lens having a positive refractive power, and a rear group (G2) is a third lens (L3) that is a single lens having a positive refractive power , A cemented lens (CL1) of a fourth lens (L4) having a positive refractive power and a fifth lens (L5) having a negative refractive power, and a sixth lens (L6) having a positive refractive power, The object side surface of the lens is a flat surface, the second lens is meniscus-shaped, and the sixth lens is cemented with an imaging device. This provides an endoscope objective optical system (1) that satisfies the following conditional expression. 4 <Fno × F6 / F1_5 <500 (1) where Fno is the effective F number of the endoscope objective optical system, F6 is the focal length of the sixth lens, and F1_5 is the distance from the first lens to the fifth lens. The composite focal length.

Description

  The present invention relates to an objective optical system, and more particularly to an endoscope objective optical system applied to a medical endoscope.

  In recent years, endoscopes are becoming smaller and higher in image quality from the viewpoint of reducing burden on patients and improving diagnosis accuracy. For this reason, high-pixel and small-sized image sensors (for example, CCD and CMOS) have been developed as image sensors for endoscopes, and the pixel pitch is becoming smaller year by year. Along with the reduction of the pixel pitch, it is required to reduce the size of the objective optical system for endoscopes while satisfying optical performance such as wide angle and aberration correction.

  By the way, in general, when the pixel pitch of the image sensor becomes small, a high-definition image cannot be obtained unless Fno is reduced to make a bright objective optical system. On the other hand, if a bright objective optical system is used, the depth is narrow and it is difficult to adjust the focus. Further, in the objective optical system, it is difficult to obtain a high-quality and high-definition image when the focus position is shifted after positioning due to an adhesive curing shift or an influence of a focus position adjusting device.

  For example, Patent Document 1 and Patent Document 2 are small-sized endoscope objective optical systems that can realize high image quality. That is, Patent Document 1 discloses an endoscope objective that is brighter by reducing Fno while having a smaller diameter, good aberrations, and strong against one blur without increasing the power of the first lens. An optical system is disclosed. Further, Patent Document 2 discloses an endoscope objective optical system with good performance of various aberrations.

Japanese Patent No. 4695662 Japanese Patent No. 4997348

However, since the endoscope objective optical system of Patent Document 1 is a bright objective optical system with a small Fno, the depth is narrow. Further, since focus adjustment is difficult and sensitivity at the focus adjustment position is strong, the influence of manufacturing variations on image quality increases, and stable production becomes difficult.
The endoscope objective optical system of Patent Document 2 has low sensitivity at the focus adjustment position, and cannot be said to be strong against manufacturing variations.

  None of the above-mentioned endoscope objective optical systems takes into account the problem of focus adjustment that occurs when the diameter is reduced. It is necessary to newly develop a high-precision focus adjustment device that does not shift the focus position. In other words, in the conventional endoscope objective optical system, design considerations for optimizing the focus adjustment are not made in the endoscope objective optical system itself.

  The present invention has been made in view of the above-described circumstances, and provides a small-diameter endoscope objective optical system that is easy to adjust in focus and can obtain a bright, wide-angle, high-definition image quality. Objective.

In order to achieve the above object, the present invention provides the following means.
One aspect of the present invention includes, in order from the object side, a front group having a negative refractive power as a whole, an aperture stop, and a rear group having a positive refractive power as a whole. In order, a first lens that is a single lens having a negative refractive power and a second lens that is a single lens having a positive refractive power, and the rear group is a single lens having a positive refractive power in order from the object side. A third lens, a cemented lens of a fourth lens having a positive refractive power and a fifth lens having a negative refractive power, and a sixth lens having a positive refractive power, and the object side surface of the first lens is An endoscope objective optical system that is flat, has a meniscus shape, has a meniscus shape, and is joined to an imaging element, and satisfies the following conditional expression.
4 <Fno × F6 / F1_5 <500 (1)
Where Fno is the effective F number of the endoscope objective optical system, F6 is the focal length of the sixth lens, and F1_5 is the combined focal length from the first lens to the fifth lens.

  According to this aspect, as the lens constituting the front group, the first lens which is a flat object having a negative refracting power on the object side is the most object side, that is, a plano-concave lens is disposed on the image side of the first lens A second lens that is a meniscus shape and a single lens having a positive refractive power is disposed. By doing this, the first lens has a retrofocus configuration, reduces water breakage during observation and cracks due to impact, corrects the aberration of the first lens, and has a convergence effect so that the lens diameter does not increase. By arranging two lenses, a compact and high-performance objective optical system suitable for an endoscope is configured.

  A rear group having a positive refractive power mainly contributing to image formation is arranged on the image side from the front group, and a positive third lens and a positive fourth lens are provided as lenses constituting the rear group. By adopting the configuration, even when Fno is small and bright, the generation of aberration is suppressed and power necessary for downsizing is distributed. In addition, since the positive fourth lens disposed on the image side in the rear group is cemented with the negative fifth lens, the positive and negative cemented lenses are disposed at positions where the peripheral ray height is high, and chromatic aberration is caused. Can be corrected. Furthermore, by arranging a positive sixth lens that is joined to the imaging element on the image side of the rear group, the optical magnification of the first lens to the fifth lens is reduced, and the sensitivity of the focus adjustment is relaxed and the assemblability is improved. Optical performance is improved.

  The endoscope objective optical system configured in this way is configured to satisfy the conditional expression (1) regarding the sensitivity at the focus adjustment position. In the conditional expression (1), when Fno × F6 / F1_5 is larger than the upper limit, when trying to obtain bright and high-definition image quality, the radius of curvature of the sixth lens increases, and as a result, at the focus adjustment position. Since the sensitivity is increased, the endoscope objective optical system is weak against focus shift. On the other hand, when the conditional expression (1) is below the lower limit, the curvature radius of the sixth lens becomes too small, so that the curvature of field becomes large and it becomes difficult to obtain good image quality. In addition, it is difficult to process the sixth lens.

In the above aspect, it is preferable that the following conditional expression is satisfied.
1.1 <SH_R1R6 <10 (2)
However, SH_R1R6 = | (R1R + R6L) / (R1R−R6L) |, where R1R is the radius of curvature of the image side surface of the first lens, and R6L is the radius of curvature of the object side surface of the sixth lens.

  By satisfying the conditional expression (2), the power balance between the first lens and the sixth lens can be appropriately maintained, and the image quality obtained by correcting the curvature of field well can be improved. Conditional expression (2) is a conditional expression related to field curvature. When SH_R1R6 is larger than the upper limit in conditional expression (2), the power balance between the first lens and the sixth lens is lost. It becomes impossible to correct well, and it becomes difficult to obtain good image quality. In addition, it becomes difficult to process the sixth lens. On the other hand, if it is smaller than the lower limit, various aberrations can be corrected satisfactorily, but the optical system is weak against defocusing.

In the above aspect, it is preferable that the following conditional expression is satisfied.
-1.5 <R4R / R6L <-0.01 (3)
Here, R4R is the radius of curvature of the image side surface of the fourth lens, and R6L is the radius of curvature of the object side surface of the sixth lens.

  By satisfying conditional expression (3), it is possible to facilitate the processing of the lens and improve the image quality of the acquired image. Conditional expression (3) is an expression related to the workability of the lens. When the upper limit of conditional expression (3) is exceeded, the edge of the fourth lens becomes too small and processing becomes difficult. On the other hand, if the value is below the lower limit, the sixth lens is too thin and difficult to process, and the curvature of field increases, making it difficult to obtain high-definition image quality.

In the above aspect, it is preferable that the following conditional expression is satisfied.
2.2 <F23 / FL <4.0 (4)
Here, F23 is the combined focal length of the second lens and the third lens, and FL is the combined focal length of the entire system.

  By satisfying conditional expression (4), manufacturing variations can be suppressed. If the lower limit of conditional expression (4) is surpassed, since the positive power is strong, the negative power of the first lens becomes strong, one-sided blur is likely to occur, and manufacturing variations are weak. If the upper limit of conditional expression (4) is exceeded, downsizing becomes difficult.

In the above aspect, it is preferable that the following conditional expression is satisfied.
−0.8 <F1 / F6 <−0.01 (5)
Here, F1 is the focal length of the first lens, and F6 is the focal length of the sixth lens.

  By satisfying conditional expression (5), it is strong in manufacturing variation and can contribute to downsizing. If the upper limit of conditional expression (5) is exceeded, it will be difficult to reduce the size, the performance of various aberrations will deteriorate, and it will be difficult to obtain good image quality. On the other hand, when the conditional expression (5) is below the lower limit, the endoscope objective optical system is weak against one-sided blur and out-of-focus and weak against manufacturing variations.

In the above aspect, it is preferable that the following conditional expression is satisfied.
0.0003 <P ² /(L×F6)<0.015 (6)
Here, P is the distance between the fifth lens and the sixth lens, and L is the total length of the endoscope objective optical system.

By satisfying conditional expression (6), the focus adjustment can be facilitated. In conditional expression (6), if the value of P ² / L is small, the focus adjustment interval cannot be taken sufficiently, and it is necessary to increase the focus sensitivity. Therefore, it is necessary to increase the value of F6. When the conditional expression (6) is below the lower limit, the optical system is susceptible to defocusing. On the other hand, when the upper limit of conditional expression (6) is exceeded, it is difficult to correct various aberrations.

In the above aspect, it is preferable that the following conditional expression is satisfied.
−2.0 <F12 / F36 <−0.6 (7)
However, F12 is a composite focal length of the front group (first lens and second lens), and F36 is a composite focal length of the rear group (from the third lens to the sixth lens).

  By satisfying conditional expression (7), the focal length of the rear group can be properly maintained. In conditional expression (7), if the upper limit is exceeded, the focal length of the rear group becomes relatively large and the image surface falls to the negative side, making it difficult to suppress various aberrations and achieving good image quality. . On the other hand, when the conditional expression (7) is below the lower limit, the focal length of the rear group becomes relatively small, the R of the rear group lens becomes small, the lens rim becomes too small, and the rear group lens is processed. Becomes difficult.

In the above aspect, it is preferable that the following conditional expression is satisfied.
0.05 <FL / L <0.12 (8)

  By satisfying conditional expression (8), it is possible to realize a reduction in size and a wide angle. In conditional expression (8), when the upper limit is exceeded, it is difficult to widen the angle, and when it is below the lower limit, miniaturization is difficult.

In the above aspect, it is preferable that the following conditional expression is satisfied.
0.06 <IH / L <0.12 (9)
However, IH is the maximum image height.

  By satisfying conditional expression (9), it is possible to reduce the size and improve the productivity. When the conditional expression (9) is below the lower limit, it is difficult to reduce the size, and when it exceeds the upper limit, the manufacturing variation is weak, so that stable production is difficult.

In the above aspect, it is preferable that the following conditional expression is satisfied.
ω> 62 ° (10)
However, ω is a half angle of view.

  By satisfying conditional expression (10), it is possible to reduce the risk of overlooking a lesion during in vivo screening. That is, by satisfying conditional expression (10), a half angle of view of 62 ° can be secured and a wide angle can be maintained.

  According to the present invention, there is an effect that it is possible to provide a small-diameter endoscope objective optical system that can be easily adjusted in focus and can obtain a bright, wide-angle, high-definition image quality.

It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on embodiment of this invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 1 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 1 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 1 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 1 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 1 of the present invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 2 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 2 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 2 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 2 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 2 of the present invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 3 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 3 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 3 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 3 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 3 of the present invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 4 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 4 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 4 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 4 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 4 of the present invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 5 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 5 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 5 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 5 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 5 of the present invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 6 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 6 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 6 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 6 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 6 of the present invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 7 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 7 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 7 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 7 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 7 of the present invention. It is sectional drawing which shows the whole structure of the endoscope objective optical system which concerns on Example 8 of this invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 8 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 8 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 8 of the present invention. It is an aberration curve figure of the endoscope objective optical system concerning Example 8 of the present invention.

An endoscope objective optical system according to an embodiment of the present invention will be described below with reference to the drawings.
As shown in FIG. 1, the endoscope objective optical system includes, in order from the object side, a front group G1 having a negative refractive power as a whole, an aperture stop S, and a rear group G2 having a positive refractive power as a whole. And.

  The front group G1, in order from the object side, is a first lens L1 having a negative refracting power and a single lens having a flat surface on the object side, a second lens L2 having a positive refracting power and a meniscus single lens, And a parallel plate F as an outer cut filter.

The rear group G2 includes a third lens that is a single lens having a positive refractive power, a cemented lens CL1 in which a fourth lens L4 having a positive refractive power and a fifth lens L5 having a negative refractive power are cemented, A sixth lens L6 that is bonded to the cover glass CG of the image sensor and has a positive refractive power is provided.
In FIG. 1, “P” shown in the rear group G2 is a focus adjustment position.

The endoscope objective optical system is configured to satisfy the following conditional expressions (1) to (10).
4 <Fno × F6 / F1_5 <500 (1)
Where Fno is the effective F number of the endoscope objective optical system, F6 is the focal length of the sixth lens, and F1_5 is the combined focal length of the first lens to the fifth lens.

  Conditional expression (1) is a conditional expression regarding the sensitivity at the focus adjustment position. In the conditional expression (1), when Fno × F6 / F1_5 is larger than the upper limit, when trying to obtain bright and high-definition image quality, the radius of curvature of the sixth lens increases, and as a result, at the focus adjustment position. Since the sensitivity is increased, the endoscope objective optical system is weak against focus shift. On the other hand, when the conditional expression (1) is below the lower limit, the curvature radius of the sixth lens becomes too small, so that the curvature of field becomes large and it becomes difficult to obtain good image quality. In addition, it is difficult to process the sixth lens.

Note that it is preferable to satisfy the conditional expression (1 ′) instead of the conditional expression (1), and further satisfy the conditional expression (1 ″) instead of the conditional expression (1) or (1 ′). Is more preferable.
6 <Fno × F6 / F1_5 <120 (1 ′)
7 <Fno × F6 / F1_5 <25 (1 ″)

1.1 <SH_R1R6 <10 (2)
However, SH_R1R6 = | (R1R + R6L) / (R1R−R6L) |, where R1R is the radius of curvature of the image side surface of the first lens, and R6L is the radius of curvature of the object side surface of the sixth lens.

  Conditional expression (2) is a conditional expression relating to field curvature, and by satisfying conditional expression (2), the power balance between the first lens and the sixth lens can be appropriately maintained, and the field curvature is favorable. It is possible to improve the image quality of an image acquired by correcting the image. In Conditional Expression (2), when SH_R1R6 is larger than the upper limit, the power balance between the first lens and the sixth lens is lost, so that the field curvature cannot be corrected well, and it becomes difficult to obtain good image quality. . In addition, it becomes difficult to process the sixth lens. On the other hand, if it is smaller than the lower limit, various aberrations can be corrected satisfactorily, but the optical system is weak against defocusing.

Note that it is preferable to satisfy the conditional expression (2 ′) instead of the conditional expression (2), and further satisfy the conditional expression (2 ″) instead of the conditional expression (2) or (2 ′). Is more preferable.
1.2 <SH_R1R6 <7 (2 ′)
2.0 <SH_R1R6 <5 (2 ″)

-1.5 <R4R / R6L <-0.01 (3)
Here, R4R is the radius of curvature of the image side surface of the fourth lens, and R6L is the radius of curvature of the object side surface of the sixth lens.

  Conditional expression (3) is an expression related to the processability of the lens. By satisfying conditional expression (3), it is possible to facilitate the processing of the lens and improve the image quality of the acquired image. If the upper limit of conditional expression (3) is exceeded, the edge of the fourth lens will be too small, making processing difficult. On the other hand, if the value is below the lower limit, the sixth lens is too thin and difficult to process, and the curvature of field increases, making it difficult to obtain high-definition image quality.

Note that it is preferable to satisfy the conditional expression (3 ′) instead of the conditional expression (3), and further satisfy the conditional expression (3 ″) instead of the conditional expression (3) or (3 ′). Is more preferable.
−1.2 <R4R / R6L <−0.05 (3 ′)
−1.0 <R4R / R6L <−0.15 (3 ″)

2.2 <F23 / FL <4.0 (4)
Here, F23 is the combined focal length of the second lens and the third lens, and FL is the combined focal length of the entire system.

  By satisfying conditional expression (4), manufacturing variations can be suppressed. If the lower limit of conditional expression (4) is surpassed, since the positive power is strong, the negative power of the first lens becomes strong, one-sided blur is likely to occur, and manufacturing variations are weak. If the upper limit of conditional expression (4) is exceeded, downsizing becomes difficult.

It is preferable to satisfy the conditional expression (4 ′) instead of the conditional expression (4), and further satisfy the conditional expression (4 ″) instead of the conditional expression (4) or (4 ′). Is more preferable.
2.2 <F23 / FL <3.7 (4 ′)
2.3 <F23 / FL <3.4 (4 ″)

−0.8 <F1 / F6 <−0.01 (5)
Here, F1 is the focal length of the first lens, and F6 is the focal length of the sixth lens.

  By satisfying conditional expression (5), it is strong in manufacturing variation and can contribute to downsizing. If the upper limit of conditional expression (5) is exceeded, it will be difficult to reduce the size, the performance of various aberrations will deteriorate, and it will be difficult to obtain good image quality. On the other hand, when the conditional expression (5) is below the lower limit, the endoscope objective optical system is weak against one-sided blur and out-of-focus and weak against manufacturing variations.

It is preferable to satisfy the conditional expression (5 ′) instead of the conditional expression (5), and further satisfy the conditional expression (5 ″) instead of the conditional expression (5) or (5 ′). Is more preferable.
−0.6 <F1 / F6 <−0.02 (5 ′)
−0.4 <F1 / F6 <−0.1 (5 ″)

0.0003 <P ² /(L×F6)<0.015 (6)
Here, P is the distance between the fifth lens and the sixth lens, and L is the total length of the endoscope objective optical system.

By satisfying conditional expression (6), the focus adjustment can be facilitated. In conditional expression (6), if the value of P ² / L is small, the focus adjustment interval cannot be taken sufficiently, and it is necessary to increase the focus sensitivity. Therefore, it is necessary to increase the value of F6. When the conditional expression (6) is below the lower limit, the optical system is susceptible to defocusing. On the other hand, when the upper limit of conditional expression (6) is exceeded, it is difficult to correct various aberrations.

Note that it is preferable to satisfy the conditional expression (6 ′) instead of the conditional expression (6), and further to satisfy the conditional expression (6 ″) instead of the conditional expression (6) or (6 ′). Is more preferable.
0.0005 <P ² /(L×F6)<0.013 (6 ′)
0.001 <P ² /(L×F6)<0.01 (6 ″)

−2.0 <F12 / F36 <−0.6 (7)
However, F12 is a composite focal length of the front group (first lens and second lens), and F36 is a composite focal length of the rear group (from the third lens to the sixth lens).

  By satisfying conditional expression (7), the focal length of the rear group can be properly maintained. In conditional expression (7), if the upper limit is exceeded, the focal length of the rear group becomes relatively large and the image surface falls to the negative side, making it difficult to suppress various aberrations and achieving good image quality. . On the other hand, when the conditional expression (7) is below the lower limit, the focal length of the rear group becomes relatively small, the R of the rear group lens becomes small, the lens rim becomes too small, and the rear group lens is processed. Becomes difficult.

Note that it is preferable to satisfy the conditional expression (7 ′) instead of the conditional expression (7), and further satisfy the conditional expression (7 ″) instead of the conditional expression (7) or (7 ′). Is more preferable.
-1.7 <F12 / F36 <-0.6 (7 ')
−1.4 <F12 / F36 <−0.6 (7 ″)

    0.05 <FL / L <0.12 (8)

  By satisfying conditional expression (8), it is possible to realize a reduction in size and a wide angle. In conditional expression (8), when the upper limit is exceeded, it is difficult to widen the angle, and when it is below the lower limit, miniaturization is difficult.

It is preferable that the conditional expression (8 ′) is satisfied instead of the conditional expression (8), and that the conditional expression (8 ″) is satisfied instead of the conditional expression (8) or (8 ′). Is more preferable.
0.06 <FL / L <0.12 (8 ')
0.07 <FL / L <0.12 (8 ″)

0.06 <IH / L <0.12 (9)
However, IH is the maximum image height.

  By satisfying conditional expression (9), it is possible to reduce the size and improve the productivity. When the conditional expression (9) is below the lower limit, it is difficult to reduce the size, and when it exceeds the upper limit, the manufacturing variation is weak, so that stable production is difficult.

Note that it is preferable that the conditional expression (9 ′) is satisfied instead of the conditional expression (9), and that the conditional expression (9 ″) is satisfied instead of the conditional expression (9) or (9 ′). Is more preferable.
0.07 <IH / L <0.12 (9 ')
0.07 <IH / L <0.11 (9 ″)

ω> 62 ° (10)
However, ω is a half angle of view.

  By satisfying conditional expression (10), it is possible to reduce the risk of overlooking a lesion during in vivo screening. That is, by satisfying conditional expression (10), a half angle of view of 62 ° can be secured and a wide angle can be maintained.

Note that it is preferable that the conditional expression (10 ′) is satisfied instead of the conditional expression (10).
ω> 65 ° (10 ')

  As described above, according to the present embodiment, the first lens L1 disposed closest to the object side as the lens constituting the front group G1 is a plane concave on the object surface side, that is, a plano-concave lens. The second lens L2 disposed on the image side of the first lens has a meniscus shape and is a single lens having a positive refractive power. With this configuration, the first lens L1 has a retrofocus configuration to reduce water breakage and cracking due to impact during observation, and the second lens increases the lens diameter while correcting the aberration of the first lens L1. Therefore, it is possible to provide a high-performance objective optical system that is suitable for an endoscope and that is small in size, small in size, bright, wide-angle, and capable of acquiring high-definition image quality.

  Then, the rear group G2 has a positive refracting power so as to mainly contribute to image formation, and as each lens constituting the rear group, a third lens L3 and a fourth lens L4, both having a positive refracting power, By providing the above, even if Fno is small and bright, the generation of aberration is suppressed and power necessary for downsizing is distributed.

Further, in the rear group G2, the positive lens and the negative cemented lens are arranged at positions where the peripheral light beam height is increased by using the fourth lens L4 arranged on the image side as a cemented lens CL1 cemented with the negative fifth lens L5. Thus, chromatic aberration can be corrected.
Furthermore, the optical magnification of the first lens L1 to the fifth lens L5 is reduced by joining the image pickup element (cover glass CG) on the image side of the rear group G2 and arranging the sixth lens L6 having a positive refractive power. In addition, the sensitivity of the focus adjustment position can be weakened.
Thereby, the focus adjustment is easy, that is, the assemblability is good and the manufacturing variation can be suppressed.

  Subsequently, Examples 1 to 8 of the wide-angle objective optical system according to the above-described embodiment will be described with reference to FIGS. In the lens data described in each example, r is a radius of curvature (unit: mm), d is a surface separation (mm), Ne is a refractive index with respect to e-line, and νd is an Abbe number.

Example 1
FIG. 2 shows an overall configuration of the endoscope objective optical system according to Example 1 of the present invention, and lens data is shown below. Aberration curves of the endoscope objective optical system according to this example are shown in FIGS. 3A to 3D.

Lens data surface number r d Ne νd
Object plane ∞ 26.7261
1 ∞ 0.4454 1.88815 40.76
2 1.3675 1.0690
3 -10.2962 1.0022 1.93429 18.90
4-5.6258 0.3341
5 ∞ 0.8909 1.49557 75.00
6 (aperture) ∞ 0.0668
7 ∞ 0.4009
8 9.4766 1.5590 1.88815 40.76
9 -3.5835 0.2227
10 3.9243 1.5590 1.69979 55.53
11 -1.9065 0.6682 1.93429 18.90
12 ∞ 0.5791
13 3.5835 1.0022 1.51825 64.14
14 ∞ 0.0223 1.51500 64.00
15 ∞ 0.7795 1.50700 63.26
16 Imaging surface

Various data Focal length 1.047
Fno 2.979
Half angle of view 66.7 °
Image height 1.000
Total length 10.60

(Example 2)
FIG. 4 shows an overall configuration of an endoscope objective optical system according to Example 2 of the present invention, and lens data is shown below. In addition, aberration curve diagrams of the endoscope objective optical system according to the present example are shown in FIGS. 5A to 5D.

Lens data surface number r d Ne νd
Object plane ∞ 24.9443
1 ∞ 0.4887 1.88815 40.76
2 1.2969 1.0452
3 3.2461 0.7126 1.93429 18.90
4.3.9951 0.3873
5 ∞ 0.8909 1.51500 75.00
6 (aperture) ∞ 0.0668
7 ∞ 0.2221
8 6.3541 1.4474 1.83945 42.71
9 -3.5783 0.2223
10 3.2639 1.7463 1.69979 55.53
11 -1.8928 0.5568 1.93429 18.90
12 ∞ 0.5738
13 3.1989 1.0022 1.51825 64.14
14 ∞ 0.0223 1.51500 64.00
15 ∞ 0.7795 1.50700 63.26
16 Imaging surface

Various data Focal length 1.092
Fno 3.019
Half angle of view 65.4 °
Image height 1.000
Total length 10.12

(Example 3)
FIG. 6 shows an overall configuration of an endoscope objective optical system according to Example 3 of the present invention, and lens data is shown below. In addition, aberration curve diagrams of the endoscope objective optical system according to the present example are shown in FIGS. 7A to 7D.

Lens data surface number r d Ne νd
Object plane ∞ 26.7261
1 ∞ 0.4476 1.88815 40.76
2 1.3211 0.8682
3 -4.7713 0.8238 1.93429 18.90
4 -3.3911 0.5791
5 ∞ 0.8909
6 (aperture) ∞ 0.0668
7 ∞ 0.3720
8 9.3689 1.6091 1.88815 40.76
9 -3.5637 0.2222
10 4.0031 1.5583 1.69979 55.53
11-1.8967 0.5587 1.93429 18.90
12 ∞ 0.5663
13 3.3105 1.0022 1.51825 64.14
14 ∞ 0.0223 1.51500 64.00
15 ∞ 0.7795 1.50700 63.26
16 Imaging surface

Various data Focal length 1.038
Fno 3.975
Half angle of view 66.5 °
Image height 1.000
Total length 10.37

Example 4
FIG. 8 shows an overall configuration of an endoscope objective optical system according to Example 4 of the present invention, and lens data is shown below. In addition, aberration curve diagrams of the endoscope objective optical system according to the present example are shown in FIGS. 9A to 9D.

Lens data surface number r d Ne νd
Object plane ∞ 26.0579
1 ∞ 0.4485 1.88815 40.76
2 1.2941 1.0110
3 ∞ 0.8909 1.51500 75.00
4 ∞ 0.2636
5 1.9246 0.6927 1.93429 18.90
6 1.7733 0.3211
7 (aperture) ∞ 0.0668
8 ∞ 0.2930
9 4.9425 1.1191 1.82017 46.62
10 -3.2375 0.2221
11 2.8952 1.5588 1.64129 55.38
12 -17817 0.5566 1.93429 18.90
13 171.5852 0.5563
14 3.1170 1.0022 1.51825 64.14
15 ∞ 0.0223 1.51500 64.00
16 ∞ 0.7795 1.50700 63.26
17 Imaging surface

Various data Focal length 1.091
Fno 3.054
Half angle of view 66.0 °
Image height 1.000
Total length 9.80

(Example 5)
FIG. 10 shows an overall configuration of an endoscope objective optical system according to Example 5 of the present invention, and lens data is shown below. In addition, aberration curve diagrams of the endoscope objective optical system according to the present example are shown in FIGS. 11A to 11D.

Lens data surface number r d Ne νd
Object plane ∞ 21.6783
1 ∞ 0.5828 1.88815 40.76
2 1.3800 1.2587
3 -3.9557 1.8182 1.85504 23.78
4 -3.6853 0.4429
5 ∞ 0.9324 1.49557 75.00
6 ∞ 0.4196
7 (aperture) ∞ 0.0699
8 ∞ 0.2331
9 8.3963 1.7716 1.83932 37.16
10 -5.4522 0.2564
11 5.2308 1.8182 1.73234 54.68
12 -1.9580 0.7459 1.93429 18.90
13 -22.1492 0.8858
14 3.4615 1.1655 1.51825 64.14
15 ∞ 0.0233 1.51500 64.00
16 ∞ 0.8159 1.50700 63.26
17 Imaging surface

Various data Focal length 0.967
Fno 2.987
Half angle of view 81.4 °
Image height 1.000
Total length 13.24

(Example 6)
FIG. 12 shows an overall configuration of an endoscope objective optical system according to Example 6 of the present invention, and lens data is shown below. In addition, aberration curve diagrams of the endoscope objective optical system according to the present example are shown in FIGS. 13A to 13D.

Lens data surface number r d Ne νd
Object plane ∞ 26.7261
1 ∞ 0.4454 1.88815 40.76
2 1.3671 0.9914
3-8.8487 1.0115 1.93429 18.90
4 -3.9093 0.3803
5 ∞
6 (aperture) ∞ 0.0668
7 ∞ 0.0668
8 9.4083 1.3763 1.88815 40.76
9 -3.5512 0.0668
10 4.8604 1.5506 1.69979 55.53
11 -1.8438 0.6561 1.93429 18.90
12 ∞ 0.5025
13 2.1158 1.0022 1.51825 64.14
14 ∞ 0.0223 1.51500 64.00
15 ∞ 0.7795 1.50700 63.26
16 Imaging surface

Various data Focal length 1.061
Fno 3.020
Half angle of view 65.1 °
Image height 1.000
Total length 9.81
Various data

(Example 7)
FIG. 14 shows an overall configuration of an endoscope objective optical system according to Example 7 of the present invention, and lens data is shown below. In addition, FIGS. 15A to 15D show aberration curve diagrams of the endoscope objective optical system according to the present example.

Lens data surface number r d Ne νd
Object plane ∞ 26.7261
1 ∞ 0.4454 1.88815 40.76
2 1.3403 1.0420
3-11.355 0.9880 1.93429 18.90
4-5.3538 0.3946
5 ∞ 0.8909 1.51500 75.00
6 (aperture) ∞ 0.0668
7 ∞ 0.6312
8 9.7005 1.5363 1.88815 40.76
9 -3.5452 0.1173
10 3.5940 1.5409 1.69979 55.53
11 -1.88859 0.6621 1.93429 18.90
12 ∞ 0.5209
13 11.1359 1.0022 1.51825 64.14
14 ∞ 0.0223 1.51500 64.00
15 ∞ 0.7795 1.50700 63.26
16 Imaging surface

Various data Focal length 1.049
Fno 3.001
Half angle of view 66.4 °
Image height 1.000
Total length 10.64

(Example 8)
FIG. 16 shows an overall configuration of an endoscope objective optical system according to Example 8 of the present invention, and lens data is shown below. FIG. 17 shows an aberration curve diagram of the endoscope objective optical system according to the present example.

Lens data surface number r d Ne νd
Object plane ∞ 26.7261
1 ∞ 0.4454 1.88815 40.76
2 1.3437 1.0364
3 -11.4435 0.9802 1.93429 18.90
4-5.2698 0.4539
5 ∞ 0.8909 1.51500 75.00
6 (aperture) ∞ 0.0668
7 ∞ 0.6343
8 9.7252 1.5315 1.88815 40.76
9 -3.5438 0.0978
10 3.5342 1.5343 1.69979 55.53
11-1.8850 0.6595 1.93429 18.90
12 ∞ 0.5155
13 22.2717 1.0022 1.51825 64.14
14 ∞ 0.0223 1.51500 64.00
15 ∞ 0.7795 1.50700 63.26
16 Imaging surface

Various data Focal length 1.058
Fno 3.003
Half angle of view 65.4 °
Image height 1.000
Total length 10.65

  In each of the above-described embodiments, the configuration in which the sixth lens and the cover glass in the image sensor are joined is described. However, there is no problem even if these are separated.

  Table 1 shows values related to the conditional expressions (1) to (10) in the endoscope objective optical systems of the first to eighth embodiments.

DESCRIPTION OF SYMBOLS 1 Endoscope objective optical system G1 Front group G2 Rear group L1 1st lens L2 2nd lens L3 3rd lens L4 4th lens L5 5th lens L6 6th lens CL1 Joint lens S Brightness stop P Focus adjustment position CG Cover Glass IMG Image plane

Claims (10)

In order from the object side, it consists of a front group having negative refractive power as a whole, an aperture stop, and a rear group having positive refractive power as a whole,
The front group includes, in order from the object side, a first lens that is a single lens having a negative refractive power and a second lens that is a single lens having a positive refractive power;
The rear group includes a third lens which is a single lens having a positive refractive power, a cemented lens of a fourth lens having a positive refractive power and a fifth lens having a negative refractive power, and a sixth lens having a positive refractive power. And consist of
The object side surface of the first lens is a plane, and the second lens is a meniscus shape,
An endoscope objective optical system in which the sixth lens is cemented with an imaging device and satisfies the following conditional expression.
4 <Fno × F6 / F1_5 <500 (1)
Where Fno is the effective F number of the endoscope objective optical system, F6 is the focal length of the sixth lens, and F1_5 is the combined focal length of the first lens to the fifth lens. The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
1.1 <SH_R1R6 <10 (2)
However, SH_R1R6 = | (R1R + R6L) / (R1R−R6L) |, where R1R is the radius of curvature of the image side surface of the first lens, and R6L is the radius of curvature of the object side surface of the sixth lens. The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
-1.5 <R4R / R6L <-0.01 (3)
Here, R4R is the radius of curvature of the image side surface of the fourth lens, and R6L is the radius of curvature of the object side surface of the sixth lens. The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
2.2 <F23 / FL <4.0 (4)
Here, F23 is the combined focal length of the second lens and the third lens, and FL is the combined focal length of the entire system. The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
−0.8 <F1 / F6 <−0.01 (5)
Here, F1 is the focal length of the first lens, and F6 is the focal length of the sixth lens. The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
0.0003 <P ² /(L×F6)<0.015 (6)
However, P is the distance between the fifth lens and the sixth lens, and L is the total length of the endoscope objective optical system. The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
−2.0 <F12 / F36 <−0.6 (7)
However, F12 is a composite focal length of the front group (first and second lenses), and F36 is a composite focal length of the rear group (third lens to sixth lens). The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
0.05 <FL / L <0.12 (8) The endoscope objective optical system according to claim 1, wherein the following conditional expression is satisfied.
0.06 <IH / L <0.12 (9)
However, IH is the maximum image height. The endoscope objective optical system according to claim 1, wherein the following conditional expression (10) is satisfied.
ω> 62 ° (10)
However, ω is a half angle of view.

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